Resonance-enhanced multiphoton ionization spectroscopy of dipeptides

We report resonance-enhanced multiphoton ionization (REMPI) spectroscopy of laser-desorbed, jet-cooled dipeptides, containing either tyrosine or pheny lalanine as an aromatic chromophore (C). The single amino acids have multip le origins that we interpret as arising from two types of conformations, wi th the carboxyl group either anti or gauche with respect to the ring. Spect ra for dipeptides of the form X-C, for example, Ala-Tyr, are similar to tho se of the corresponding single amino acid. On the other hand, spectra for d ipeptides of the form C-X, for example, Tyr-Ala, show significant changes i n the peaks, which we associate with gauche conformations. This observation can be understood in terms of an interaction between the carboxyl terminus and the ring, associated with the molecule assuming a folded conformation.